The growing ubiquity of carbon nanotubes (CNTs) has raised serious concerns about the potential human health and environmental implications of ultrafine particles in general, and manufactured nanomaterials in particular. To this end, while significant research is conducted to ascertain the material toxicity of nanotubes and their potential, for example, to stimulate pulmonary fibrogenesis, aerosol dynamics simulation tools can serve as a valuable complementary asset to investigate the link between the respiratory flow, the ensuing dynamics of nanotubes in the respiratory tracts, and the eventual deposition of CNTs on the respiratory system walls. A typical outcome of such an analysis would be an estimate of how deep, and at what concentration levels and distributions, CNTs of various sizes and types would penetrate in the respiratory tract under physiologic flow conditions. At the heart of a successful nanotube dynamics simulation lies the availability of accurate models for forces experienced by the nanotube. The dominant forces in the nanometer scale are drag and Brownian motion, where the former is the focus of this proposal. An order of magnitude analysis of the length scales reveals that, depending on the size and orientation of the CNT with respect to the freestream, there may be two flow regimes of relevance to CNT drag - free-molecule and transition. Yet, a review of the literature reveals an absence of drag formulae for nanotubes in these flow regimes. To address this critical and unmet CNT dynamics modeling need, the Specific Aim proposed for this project is to perform a series of Molecular Dynamics simulations of air flow over CNTs under physiologically realistic conditions, and to obtain the drag on the CNT as a function of the relative humidity of the respiratory tract; the flow rate; the inclination angle of the CNT with respect to the streamwise direction; as well as the CNT diameter, chirality, aspect ratio, and end effects. The simulation data will be analyzed and reduced into a drag coefficient function. The results will be presented at an aerosol-related conference and subsequently submitted to (aerosol) journals for publication so as to make the drag coefficient function available to the wider scientific community; thereby, enabling future aerosol dynamics simulations of CNT flow in dry or moist air. PUBLIC HEALTH RELEVANCE: The ability to accurately predict the transport and deposition details of carbon nanotubes in the human respiratory tract can be of significant benefit to making successful toxicologic impact assessments, establishing appropriate environmental standards, and determining safe exposure limits. The capability to make such predictions hinges, as a minimum, on the availability of accurate drag coefficients for carbon nanotubes under physiologically correct flow conditions. Yet, such drag coefficient data for nanotubes are absent in the literature. The objective of this research is to fill this critical gap, and to develp drag coefficient data using results from a series of simulations, which will be conducted during the project.